The present invention relates to optical systems for forming an image of an object with extremely high resolution such as, for example, optical projection systems used for exposing a semiconductor wafer to the image of a mask in microcircuit fabrication.
As perhaps in no other field than microcircuit optical lithography has the design of optical systems presented such a challenge and necessitated such innovative and unorthodox optical configurations to achieve the utmost in image fidelity. In U.S. Pat. No. 3,748,015 to A. Offner, there is described a catoptric system producing a restricted annular field. While the system is highly corrected against optical aberrations, the narrowness of the annular field is, of course, a disadvantage and to overcome this, Offner has devised optical systems comprising refractive elements which produce wider fields. Such optical systems are described and claimed in U.S. Pat. No. 4,293,186 issued Oct. 6, 1981 on co-pending application Ser. No. 106,415 filed Dec. 21, 1979, the contents of which are incorporated herein by reference.
Of equal importance with image quality in optical microlithography and perhaps more difficult to accomplish is the generation of images in which each image element occupies the same position, to within a few micro-inches, as the corresponding point in the object field. For example, a specific embodiment of the optical system shown in FIG. 10 of the aforementioned U.S. Pat. No. 4,293,186 designed with unit power and an instantaneous field of about five square centimeters can, with UV illumination in the range of 2400 to 2800 Angstroms, forms high contrast images of submicron features so that the instantaneous field (which is scanned across the image plane to effect wafer exposure) contains more than 10.sup.8 bits or picture elements. In the nominal system the magnification and distortion are such that each of these bits is in its desired location to an accuracy of less than one microinch. To achieve these results the projection system includes six reflecting surfaces and eight refracting surfaces. If each of these is fabricated to tolerances which are at the limit of the present state of the art and the system is aligned as well as can be assured, it can be shown that a considerable degradation of the contrast of the imagery and intolerable changes in relative locations of the images over the field are to be expected due to the limits of achievable accuracy in the manufacture of components and in assembly and alignment thereof.
So, even as the need for near-perfection in the formation of images in the field of optical microlithography has pushed the lens designer's art to the utmost, so has the practical implementation of such designs in operative optical hardware imposed tolerances in the fabrication, assembly and alignment of optical elements which are practically impossible to meet in volume production. Thus, it is of little consequence that the theoretical design of an optical system is capable of diffraction limited performance and of reproducing printed circuits with line widths and/or line spacings of one micron or less when the magnification of the system may depart from nominal by intolerable amounts, i.e., a few parts per million, due to incorrect spacing between elements or inaccurate measurements of parameters of the optical components.
It is to this problem that the present invention is directed and it is a general object of that invention to provide a method of, and means for, relaxing the tolerances on the manufacture of optical components and on their assembly and alignment in the production of apparatus for projection optical microlithography and other optical systems requiring similar degrees of precision in operation.